Cone-Beam Computed Tomography Assessment of Quality of Endodontic Treatment and Prevalence of Procedural Errors in Mandibular Molars

Objectives This study assessed the quality of endodontic treatment and the prevalence of procedural errors in permanent mandibular molars using cone-beam computed tomography (CBCT). Materials and Methods This cross-sectional study was conducted on 328 CBCT scans (182 females and 146 males) of endodontically treated mandibular molars retrieved from the archives of two radiology centers in Ardabil city, Iran, in 2019. Mandibular molars were evaluated on sagittal, coronal, and axial sections regarding obturation length, obturation density (voids), missed canals, broken instruments, apical perforation, strip perforation, ledge formation, transportation, root fracture, root resorption, and periapical lesions by a senior dental student under the supervision of an oral and maxillofacial radiologist and an endodontist. Differences between the frequency of procedural errors and tooth type and gender were analyzed by the chi-square test. Results The frequency of underfilling, missed canals, overfilling, voids, apical perforation, transportation, ledge formation, broken instruments, root fracture, strip perforation, root resorption, and periapical lesions was 34.8%, 17.4%, 16.8%, 14.3%, 7.3%, 6.1%, 4.3%, 3%, 1.2%, 0.6%, 5.5%, and 46%, respectively. The frequency of root fracture was significantly higher in females than in males (P < 0.05). The prevalence of underfilling was the highest in right second molars (47.2%), followed by right first molars, left second molars, and left first molars (P < 0.005). The frequency of transportation was maximum in right first molars (10%), followed by right second molars, left first molars, and left second molars (P < 0.04). Conclusion Underfilling, missed canals, and overfilling were the most prevalent procedural errors in mandibular molars in our study population.


Introduction
Eradication of bacteria from the root canal system is the key to a successful endodontic treatment [1]. Adequate elimination of bacteria and prevention of their recolonization can increase the success of endodontic treatment to 94% [2,3]. A successful endodontic treatment requires optimal cleaning and shaping of the root canal to the working length while maintaining the original canal path, followed by complete obturation of the root canal system and the creation of a hermetic seal.
Te success rate of endodontic treatments performed by general dentists is reportedly 60% to 75% [4]. Procedural errors are among the main factors responsible for the suboptimal success rate of endodontic treatments performed by general dentists [5,6]. Procedural errors can adversely afect the treatment prognosis and may not be easily corrected. Some procedural errors may require endodontic retreatment, apicoectomy, or even tooth extraction. Root canal transportation, apical perforation, strip perforation, access cavity perforation, and instrument fracture are among the commonly occurring procedural errors [7]. Obturation errors such as underflling and overflling, sealer extrusion and void formation are also common [8]. Working length signifcantly afects the treatment outcome, such that root fllings shorter than the apex by 2 mm decrease the success rate to 68% to 77%, and overflled canals exceeding the apex have a success rate of approximately 75% [9,10]. Root perforation causes infection of the periodontal ligament and alveolar bone and can lead to endodontic treatment failure [11]. Also, a signifcant correlation exists between instrument fracture in the canal and poor treatment outcome [12].
Endodontic treatment of molar teeth is more challenging, and requires more caution compared with anterior and premolar teeth, and is associated with a higher rate of procedural errors. Te reason is their difcult accessibility and anatomical complexities [13]. Mandibular molar teeth have the highest rate of ledge formation [14].
Te quality of root canal treatment is routinely assessed by periapical radiography [15,16]. However, periapical radiography provides a 2D image of 3D root canal anatomy, and cannot reveal the details of endodontic treatment. CBCT has been designed to generate accurate three-dimensional images of teeth and their adjacent tissues. Tis is typically accomplished with a signifcantly lower efective dosage than traditional medical computed tomography [17]. Cone-beam computed tomography (CBCT) enables 3D assessment of the quality of root fllings (voids, underflling, overflling, and extrusion of sealers) [8]. It is also valuable for the assessment of the complex morphology of molar teeth and can greatly help in cases where the conventional modalities fall short [18][19][20]. Periapical disease may be detected sooner using CBCT compared with periapical views, and the true size, extent, nature, and position of periapical and resorptive lesions can be assessed [21]. Te CBCT is also a highly advantageous educational instrument. With the aid of CBCT, the quality of endodontic treatments and iatrogenic errors can be determined utilizing existing archives that reveal the weak points of clinicians, allowing for target-based learning to reduce these weaknesses [22].
In general, root canal therapy includes four steps: diagnosis, access cavity preparation, cleaning and shaping, and obturation. Te success of each step depends on the correct implementation of the previous step. Mastering all four steps and having adequate knowledge about the possible procedural errors that may occur in each step can improve the overall quality of the procedure. Accordingly, knowledge about the most common procedural errors is imperative to prevent their occurrence [23,24]. Tis study aimed to assess the quality of endodontic treatment and the prevalence of procedural errors in permanent mandibular molars using CBCT.

Materials and Methods
Tis cross-sectional study evaluated 328 CBCT scans (182 females and146 males) of endodontically treated mandibular molars (n � 328) retrieved from the archives of two radiology centers in Ardabil city, Iran, in 2019. Te CBCT scans had been taken for diagnostic and treatment planning purposes not related to this study. Te sample size was calculated according to Morgan's table. Te study was approved by the ethics committee of Ardabil University of Medical Sciences (IR.ARUMS.REC.1398.155). Te patients consented to the use of their CBCT scans for research purposes at the time of radiography.
Te inclusion criteria were high-quality CBCT scans visualizing endodontically treated mandibular molars. Mandibular molars with prosthetic crowns, intracanal posts, and deep restorations were excluded due to artifact generation. Te CBCT scans were selected by convenience sampling.
A trained and calibrated senior dental student evaluated the endodontically treated mandibular molars on CBCT scans under the supervision of an oral and maxillofacial radiologist and an endodontist. Mandibular molars were evaluated in the sagittal, coronal, and axial planes regarding the length of root flling, density of root flling (presence of voids), number of missed canals, presence of broken instruments, apical perforation, strip perforation, ledge formation, transportation, root fracture, root resorption, and presence of periapical lesions.
To ensure the validity of assessments, 25% of the CBCT scans were randomly selected and re-evaluated by the supervising oral and maxillofacial radiologist and endodontist. Also, to assess intraexaminer reliability, all CBCT scans were evaluated by the senior dental student 10 days after their primary assessment, and the agreement between the fndings in the frst and second observations was calculated using Cohen's Kappa, yielding perfect agreement (Kappa � 1).
SPSS version 25 was used to analyze the data. Te frequency and percentage of procedural errors (underflling, overflling, obturation density, missed canals, broken instruments, apical perforation, strip perforation, ledge formation, transportation, root fracture, root resorption, and periapical lesions) were calculated and reported in general and separately for diferent canals of mandibular right and left frst and second molars. Te frequency distribution of procedural errors based on gender and tooth type (right/left frst/second molars) was analyzed by the chi-square test at the 0.05 level of signifcance.
Of all patients, the age of 13 had not been disclosed in their records. Te mean age of the remaining 315 patients was 41.22 ± 11.84 years (range 15 to 74 years).
Te frequency of underflling, missed canals, overflling, voids, apical perforation, transportation, ledge formation, broken instruments, root fracture, strip perforation, root resorption, and periapical lesions was 34.8%, 17.4%, 16.8%, 14.3%, 7.3%, 6.1%, 4.3%, 3%, 1.2%, 0.6%, 5.5%, and 46%, respectively. Table 1 presents the frequency of underflling, overflling, and voids in diferent canals of endodontically treated mandibular molars. As shown, underflling had the highest frequency in the mesiobuccal canals (68.4%), followed by the mesiolingual canals (64.9%). Overflling had the highest frequency in the distal canals (56.4%), followed by the mesiolingual canals (40%). Te highest frequency of voids was noted in the mesiolingual canals (57.4%), followed by the mesiobuccal canals (42.6%). Table 2 presents the frequency of missed canals, ledge formation, and apical perforation in diferent canals of endodontically treated mandibular molars. Te distolingual canal (40.4%) was the most commonly missed canal, followed by the mesiobuccal canal (24.6%). Ledge formation had the highest frequency in mesiolingual canals (57.1%), followed by mesiobuccal canals (42.9%). Apical perforation had its maximum frequency in the distal canals (58.3%), followed by the mesiobuccal canals (33.3%). Table 3 indicates the frequency of canal transportation, root resorption, and apical lesions in diferent canals of endodontically treated mandibular molars. As shown, the distal canals had the highest frequency of canal transportation (50%), followed by the mesiobuccal canals (40%). Root resorption had maximum frequency in the distal canals (61.1%), followed by the mesial canals (55.6%). Apical lesions had the highest frequency in the mesial canals (82.1%), followed by the distal canals (56.3%). Table 4 compares the frequency of procedural errors in males and females. Te chi-square test showed that the frequency of root fracture was higher in females than males (P � 0.025). No other signifcant diferences were noted (P > 0.05). Table 5 compares the frequency of procedural errors in the right and left mandibular frst and second molars. A signifcant diference was noted in the frequency of underflling among diferent tooth types (P < 0.005) such that its frequency was maximum in right second molars (47.2%) and minimum in left frst molars (20%). Te frequency of missed canals was also signifcantly diferent among diferent teeth (P � 0.010) such that right second molars had the highest frequency of missed canals (30.6%), while left frst molars had the lowest frequency of missed canals (12.5%). Te frequency of canal transportation was also signifcantly diferent among diferent teeth (P � 0.037) such that its frequency was maximum in right frst molars (10%) and minimum in left second molars (1.2%). No other signifcant diferences were found (P > 0.05).    Table 6 shows the frequency of root canal morphology of mandibular molars, according to the Vertucci's classifcation. As shown, the most common root morphology was type IV and II in mesial root of frst molars, I and II in distal root of frst molars, IV and II in the mesial root of second molars, and I in distal root of second molars.

Discussion
Tis study assessed the quality of endodontic treatment and the prevalence of procedural errors in permanent mandibular molars using CBCT. CBCT has been designed to generate accurate three-dimensional images of teeth and their adjacent tissues. Tis is typically accomplished with a signifcantly lower efective dosage than traditional medical computed tomography [17]. In response to the rising demand for safer, more predictable treatments, numerous professionals have adopted CBCT to enhance visualization and comprehension in complex clinical settings [25,26]. By reconstructing images in 3D, CBCT can aid in the detailed evaluation of structures' anatomy and morphology. CBCT images always aid in locating a larger number of roots or canals than conventional techniques [27]. Notably, mandibular molars exhibit a high degree of variety in canal confgurations (Table 6).
CBCT is also helpful prior to periapical surgery; the size of the cortical and the relationship of anatomical structures like the maxillary sinus and inferior dental nerve to the root apices shall be evaluated using CBCT scans [17]. CBCT images are useful for analyzing treatment outcomes too. CBCT can detect periapical disease earlier than periapical views, and the true size, extent, character, and position of periapical and resorptive lesions can be evaluated [21]. Te CBCT is also a highly advantageous educational instrument. With the aid of CBCT, the quality of endodontic treatments and iatrogenic errors can be determined utilizing existing archives that reveal the weak points of clinicians, allowing for target-based learning to reduce these weaknesses [22].
Hendi et al. [28] evaluated the procedural errors in endodontic treatments performed by dental students in Hamadan, Iran, and reported that apical transportation, ledge formation, and apical perforation were more common in molar teeth. Also, AlRahabi [29] reported the maximum frequency of errors in mandibular molars (43.1%). Te higher frequency of errors in molar teeth is due to their difcult accessibility and anatomical complexities. Accordingly, mandibular molars were evaluated in this study.
Te present results revealed that the prevalence of periapical lesions was 46%. Such a high rate is probably related to poor quality of chemomechanical preparation of   [30] reported that although the quality of the coronal restoration has an infuence on the treatment outcome, the quality of the endodontic treatment was the most important factor for the success of root canal treatment and the absence of a periapical lesion. AlRahabi [29] evaluated the technical quality of endodontic treatments and iatrogenic errors by undergraduate dental students in a dental school in Saudi Arabia and reported optimal technical quality in 68.9% of the teeth. Eskandarloo et al. [31] evaluated the technical quality of endodontic treatments performed by 5th year dental students by radiography in Hamadan, Iran, and reported that the technical quality of root fllings was acceptable in only 10.4% of the cases. Awooda et al. [32] radiographically evaluated the technical quality of endodontic treatments performed by dental students in Sudan and reported that the overall quality was optimal in 55.5% of the cases. In the current study, the incidences of periapical lesions was marginally higher in males and on the right side of the mandible, but these diferences were not statistically signifcant. Alghamdi and Almehmudi [33] also reported that the prevalence of periapical lesion was signifcantly associated with male gender and the right mandibular side, comparable to our fndings. In the present study, underflling (34.8%) was the most frequent procedural error, followed by missed canals (17.4%), overflling (16.8%), and voids (14.3%). Te results of previous studies have been variable regarding the most frequent procedural errors. In the study by AlRahabi [29], the frequency of underflling and overflling was 49.9% and 24.1%, respectively. Tese values were 17.1% and 12%, respectively, in the study by Eskandarloo et al. [31], 17.8% and 10.2%, respectively, in a study by Ehsani et al. [34] on the quality of endodontic treatments performed by undergraduate dental students in Babol, Iran, and 34.5% and 4.2%, respectively, in a study by Barrieshi-Nusair et al. [35] on dental students in Jordan. Te frequency of overflling was 18.2% by undergraduate dental students in a study by Haji-Hasani et al. [36] in Qazvin, Iran. Te frequency of underflling and overflling was 37.45% and 6.25%, respectively, in a study by Jamani and Fayyad [37] in Jordan. Tese rates were 23.3% and 15.3%, respectively, in a study by Mozayeni et al. [38] in a dental school in Tehran, Iran, 21% and 9%, respectively, in a study by Lynch and Burke [39], and 37.3% and 7.8%, respectively, in a study by Ilgüy et al. [5] on dental students in Turkey. Nouroloyouni et al. [22] also reported that underflling was the most common error in the second and frst mandibular premolars (9.5% compared with 9.2%), respectively. In addition, overflling was the second most common error in this study (6.3%). Another study reported the frequency of underflled and overflled root fllings to be 10.5% and 5.42%, respectively [40]. Variations in the reported values can be attributed to diferent methodologies, defnitions (e.g. the acceptable length of root fllings), instrumentation and obturation techniques, experience and expertise of clinicians, patient cooperation, quality of assessment, type and quality of radiographs used to judge the quality of treatments, diferent quality of instructions, and diferences in sample size and tooth type. Moreover, it should be noted that underflling and overflling are often secondary to incomplete or incorrect implementation of previous steps.
Te present results showed that the frequency of voids was 14.3%. Tis value was 12.6% in the study by AlRahabi [29] and 50.9% in the study by Haji-Hasani et al. [36] 52.7% in the study by Ilgüy et al. [5], 10% in the study by Lynch and Burke [39], and 27.3% in the study by Mozayeni et al. [38]. Te presence of voids indicates incomplete root flling, and adversely afects the treatment prognosis. Te presence of voids in the middle and apical thirds of the root canals has a poorer prognosis than voids in the coronal third [41]. Te reason for void formation is inadequate access to all parts of the root, or its nonconical shape, preventing the access of condensing instruments to the apical region in lateral compaction and vertical condensation techniques [42].
Te frequency of ledge formation was 4.3% in the present study. Tis value was 6.54% in a study by Zambon da Silva et al. [43] on procedural errors by dental students in Brazil, 2.8% in a study by Vukadinov et al. [44] on procedural errors by dental students in Serbia, 14% in a study by Balto et al. [45] on the performance of dental students in Saudi Arabia, and 17.5% in a study by Dadresanfar et al. [46] on the quality of endodontic treatments by dental students in Tehran, Iran. Te prevalence of ledge formation was 24.8% in a study by Eleftheriadis and Lambrianidis [14] on dental students in Greece. Tis rate was 26% in a study by Mozayeni et al. [38] and 55% in a study by Khabbaz et al. [47]. In some cases, transportation and apical perforation are considered as deep ledges, which can afect the reported frequency rates for ledge formation. Te quality of instruction of dental students and the obturation technique may also be responsible for the variations in the reported frequency rates for ledge formation because it has been reported that the passive stepback and balanced force techniques can minimize the risk of ledge formation and transportation [42]. Canal curvature is the main factor responsible for ledge formation and  [48]. Te incorrect form of the access cavity, faulty detection of the canal path, incorrect working length determination, and not using irrigants are among other contributing factors to ledge formation [42].
In the current study, apical perforation, canal transportation, broken instruments, root fracture, and strip perforation had a frequency of 7.3%, 6.1%, 3%, 1.2%, and 0.6%, respectively. Te relatively low frequency of the aforementioned errors was in agreement with the results of other studies in this respect. Te frequency of root perforation was 1.1% in the study by Jamani and Fayyad [37]. Apical transportation and perforation had 8.7% and 0.7% frequency rates, respectively, in the study by Mozayeni et al. [38], while Lynch and Burke [39] did not fnd any evidence of broken instrument or root perforation in their study population. Te frequency of broken instruments was 2.5% in the study by Ilgüy et al. [5], while this rate was 9.2% in the study by AlRahabi [29]. Te frequency values for apical perforation and canal transportation were both 2.3%. Te frequency of broken instruments, missed canals, and apical transportation was 2.8%, 0.3%, and 0.3% in the study by Vukadinov et al. [44]. Apical transportation and root perforation both had a frequency of 7% in the study by Balto et al. [45]. Te frequency of root perforation was 2.7% in the study by Eleftheriadis and Lambrianidis [14].
Te frequency of strip perforation was very low in the present study. Tis procedural error may occur due to the use of Gates Glidden drills in the danger zone (root walls close to the furcation area) [14].
In the present study, underflling and missed canals had a signifcantly higher frequency in right second molars, and transportation had a signifcantly higher frequency in right frst and second molars. Also, the frequency of root fracture was higher in females than males (P � 0.025). No other signifcant diferences were noted (P > 0.05). Alnowailaty et al. [49] also found that there are more missed canals in females, similar to our fndings; however, they found that the frequency of missed canals is highest in the mesiobuccal canals of the frst molars. Tis may be due to the fact that the quality of treatment is largely dependent on the experience and skill of the clinician and the fact that access to the posterior teeth is more difcult in female patients, which may be the cause of more untreated canals.
Te use of CBCT for the assessment of procedural errors was a strength of this study due to the high accuracy of this modality for this purpose. De Alencar et al. [50] showed the superior efcacy of CBCT compared with periapical radiography for the detection of endodontic procedural errors.
Further multicenter studies on a larger sample size are required to compare the frequency of procedural errors between general dentists and endodontists. Also, the role of infuential factors in the occurrence of procedural errors should be further scrutinized.

Conclusion
Underflling, missed canals, and overflling were the most prevalent procedural errors in mandibular molars in our study population.

Data Availability
Te data used to support the fndings of this study are available from the corresponding author upon reasonable request up to one year after the publication date.

Conflicts of Interest
Te authors declare that there are no conficts of interest regarding the publication of this paper.